1. Field of the Invention
The present invention relates to an earth electrode apparatus and, more particularly, to an earth electrode apparatus for conductive contact of an earth electrode with a workpiece.
2. Description of the Related Art
In related art, direct spot welding or series welding is used for the welding of a workpiece made up of plate members laid one on top of the other. A large electric current is applied for a certain period to the workpiece sandwiched between a pair of weld electrodes while applying welding pressure thereto. A nugget is formed due to heat generated at a weld region, thereby welding the plate members together.
In spot welding of plate members in a structure such as an automobile body, there is no space for providing one of a pair of weld electrodes for direct spot welding or providing a back electrode for series welding, in some cases.
One-sided resistance spot welding, which enables a workpiece made up of plates stacked in layers to be welded from one side only, is used in such a case.
An example of one-sided resistance spot welding will now be explained with reference to FIG. 9. To weld a workpiece 100 made up of a second plate member 102 and a first plate member 101 laid on the second plate member 102 by using a one-sided resistance spot welding method, an earth electrode 106 is brought into contact with the second plate member 102 in a conductive manner; a weld electrode 105 is brought into contact with the surface of the first plate member 101 to apply welding pressure thereto. The first plate member 101 and the second plate member 102 are in contact with each other at a joint “a” in a conductive manner. A welding current application route X, which leads from the weld electrode 105 to the earth electrode 106 by way of the first plate member 101, the joint a, and the second plate member 102, is formed.
In the state in which the first plate member 101 is in conductive contact with the second plate member 102 at the joint a, a welding current “i” is applied from the weld electrode 105 to the earth electrode 106. A part “ia” of the welding current i flows through the welding current application route X. As a result, a part of each of the first plate member 101 and the second plate member 102 at the joint a melt to form into a nugget N. The welding current i flowing from the weld electrode 105 to the earth electrode 106 includes another part “ib”. The another part ib flows, for example, through a current application route X1 leading from the first plate member 101 to the earth electrode 106 via an already-welded joint “b” and then via the second plate member 102 as a branch current without flowing through the joint a.
In such one-sided resistance spot welding, a copper plate that offers a relatively large area of contact with the second plate member 102 is generally used as the earth electrode 106, which is brought into conductive contact with the second plate member 102, to avoid heat generation due to conductive resistance between the second plate member 102 and the earth electrode 106. In actuality, however, when the earth electrode 106 made of a rigid copper plate is in contact with the second plate member 102 made of a steel plate, the second plate member 102 and the earth electrode 106 tend to be in a point-contact state, in which they are locally in contact because of, for example, variation in the contact angle of the earth electrode 106 with respect to the second plate member 102, or the surface irregularity of the second plate member 102. Due to point contact, current density is high at the region of contact. Heat generated at the region might melt and thus damage the second plate member 102 or the earth electrode 106 locally.
As illustrated in FIG. 10, in which the same numerals and symbols are assigned to members that are the same as those illustrated in FIG. 9 to omit the explanation thereof, if a commonly-used weld electrode is used as an earth electrode 107, the area of contact of the second plate member 102 and the earth electrode 107 is small. Therefore, current density is high at the region of contact. Heat generated at the region damages the second plate member 102 and increases the wear of the earth electrode 107.
As illustrated in FIG. 11, in which the same numerals and symbols are assigned to members that are the same as those illustrated in FIG. 9 to omit the explanation thereof, if an earth electrode 108 made of flexible braided copper wires is used to make the area of contact with the second plate member 102 large, it is possible to reduce current density at the region of contact, thereby preventing the second plate member 102 from being damaged due to melting. However, the earth electrode 108 is susceptible to deformation and wear. If the earth electrode 108 deforms or wears due to use, the area of contact of the second plate member 102 and the earth electrode 108 becomes smaller, which increases current density.
Moreover, if copper element wires of the earth electrode 108 become damaged, there is a risk that the damaged element wires might melt in contact with the second plate member 102 and become stuck thereto, which degrades the quality of the workpiece.
To provide a solution to the above problems, an earth connection apparatus that can secure the area of contact with a workpiece has been proposed in the art as disclosed in Japanese Unexamined Patent Application Publication No. 2004-136292. With reference to FIG. 12, the schematic structure of such an earth connection apparatus will now be explained.
An earth connection apparatus 110 includes a cylinder unit 112 attached to a robot arm 111, a bracket 115 fixed to a piston rod 113 extending downward from the cylinder unit 112 by means of nuts 114, cushion units 120 hanging from the left and right parts of the bracket 115 respectively, a pair of L-shaped metal clasps 116 each of which is attached to the lower end of the corresponding cushion unit 120, and an earth electrode 130 having the shape of a curved laminated spring. The ends of the earth electrode 130 are fixed to the respective L-shaped metal clasps 116 by means of bolts. The earth electrode 130 is made of a plurality of thin conductive strip plates stacked in layers. They have been embowed into the shape of a curved laminated spring in advance. An earth cable 131 is connected to the earth electrode 130.
The cushion unit 120 includes a flanged bush 121 inserted in the bracket 115 from above, a rod 122 inserted through the flanged bush 121 as a vertically-movable sliding rod, a male screw 123 integrally formed at the upper end of the rod 122, a stopper 124 fitted in the male screw 123, a nut 125 for fixing the stopper 124 to the rod 122, springs 126 arranged around the rod 122, a retainer 127 for supporting the lower end of the springs 126, an insulation bush 128 fitted in the L-shaped metal clasp 116 to receive the lower surface of the retainer 127, and a bolt 129 for fixing the L-shaped metal clasp 116 to the rod 122 through the insulation bush 128.
When an upward force that is greater in magnitude than the opposing force of the springs 126 is applied to the L-shaped metal clasp 116, the springs 126 are compressed. The rod 122 moves upward with respect to the bush 121 to raise the stopper 124 and the nut 125 away from the bush 121. Therefore, the L-shaped metal clasp 116 comes closer to the bracket 115.
The earth electrode 130 is positioned to face the surface of a workpiece by means of the robot arm 111. When the earth electrode 130 is brought into contact with, and pressed against, the workpiece, the earth electrode 130, which has the shape of a curved laminated spring, deforms according to the surface shape of the workpiece. In this way, the area of contact of the workpiece and the earth electrode 130 is secured, which makes it possible to reduce contact resistance, that is, conductive resistance, between the workpiece and the earth electrode 130.
To sum up, as an effect produced by the earth connection apparatus 110 disclosed in Japanese Unexamined Patent Application Publication No. 2004-136292, which includes the cylinder unit 112 attached to the robot arm 111 and further includes the curved-laminated-spring-type earth electrode 130 supported indirectly by the piston rod 113 extending downward from the cylinder unit 112 with the bracket 115 and the cushion units 120 provided therebetween, it is possible to cause the earth electrode 130 to deform according to the surface shape of a workpiece and thereby to secure the area of contact of the workpiece and the earth electrode 130 by positioning the earth electrode 130 face to face with the surface of the workpiece by means of the robot arm 111 and then bringing the earth electrode 130 into contact with, and pressing it against, the workpiece.
However, since the curved-laminated-spring-type earth electrode 130, which is fixed indirectly to the cushion units 120 at respective ends as if suspending therefrom with the respective L-shaped metal clasps 116 provided therebetween, is pressed against the surface of a workpiece to utilize its deformation for securing the area of contact, the earth electrode 130 has to be elongated and large. Besides, the earth connection apparatus 110 occupies a large space for operation. Therefore, a workpiece with which the earth electrode 130 is brought into contact and the shape thereof is limited.
Moreover, since the earth electrode 130 is pressed against the surface of a workpiece to utilize its deformation for securing the area of contact, a large load has to be applied for pressing. Furthermore, because of variation in the contact angle of the earth electrode 130 with respect to a workpiece, especially because of variation in the contact angle of the earth electrode 130 in the direction of the width thereof, the workpiece and the earth electrode 130 tend to be in a so-called line-contact state, in which the earth electrode 130 is in contact with the workpiece locally at its edge only. Due to line contact, current density is high at the region of contact. Therefore, the workpiece or the earth electrode 130 might be damaged locally due to heat generated at the region.